**2.2 Anaerobic digestion principle**

Among the various techniques of stabilization, anaerobic digestion, or methanisation, is the most interesting one. Indeed, according to Suh and Roussaux (Suh & Roussaux, 2002), it is the least aggressive treatment, towards to the environment. The anaerobic micro-organisms

For the treatment of the different pollution types, vvarious techniques and processes of different chemical, biological and physico-chemical natures as well as a coupling of the last two, are developed. The treatment and the final elimination consist of a sequence of unit operations with a great number of possible options among which the best one is to be chosen, taking into account the upstream (nature, characteristics, and waste quantities) and

The present study is more concerned by the biodegradable organic solid wastes which are characterised by a high organic matter concentration, recommanding a biological treatment. One of technologies to carry out the treatment of the organic fraction of this organic waste is anaerobic digestion (bio-methanization, this process is presented with more details in the next sections of this chapter), which consists of a biological degradation in an anaerobic phase of the organic matter into biogas with a high methane percentage. This technology is becoming essential in the reduction of organic waste volume and the production of biogas, a renewable source of energy. It can be used in a variety of ways, with a heating value of approximately 600 -800 Btu/ft and a quality that can be used to generate electricity, used as fuel for a boiler, space heater, for refrigeration equipment, or as a cooking and lighting fuel.

The use of anaerobic digestion for the treatment and the stabilization of solid waste is not new. It had been used in the 19th century. In rural parts of China and India, simple reactor constructions were used a long time ago to treat the manure and agricultural wastes in order to recover energy for cooking and lighting (Gijzen, 2002). In 1860s in France (McCarty, 2001), the anaerobic digestion of sludge waste was obtained from wastewater treatment plant, on a large scale, by means of an advanced technology. Furthermore, at the end of 1980s, codigestion processes treating a mixture of different types of waste, were introduced (Ahring, 2003). Today, anaerobic digestion is one of the most environmentally friendly and suitable treatment methods for of solid organic waste. This technology is widely applied for bioenergy production, because of the increasing request for renewable energy. A consequence of the increasing implementation of this technology is the necessity to determine the

Among the various techniques of stabilization, anaerobic digestion, or methanisation, is the most interesting one. Indeed, according to Suh and Roussaux (Suh & Roussaux, 2002), it is the least aggressive treatment, towards to the environment. The anaerobic micro-organisms

ultimate biogas potential for several solid substrates (Angelidaki & al., 20096).

downstream (local possibilities of final eliminations) constraints as well as the cost.

eventual stabilising treatment.

**2. Anaerobic digestion process 2.1 Anaerobic digestion historical** 

**2.2 Anaerobic digestion principle** 

mineralised organic matter. The sludge characterisation is essential for the choice of the most adequate treatment method as well as for the prediction of each treatment stage performance. Generally distinction is made between primary sludges which are recovered by simple waste waters decantation, and are of high concentrations in mineral and organic matter, and the biological or secondary sludges resulting from a biological treatment of waters. These latter have different compositions, depending on the nature of the degraded substrate, the operation load of the biological reactor and the use organic pollution (biodegradable organic matter) as substrate to produce biogas which can be exploited according to several forms. Thus, anaerobic digestion allows a reduction of the dry matter from approximately 50% (OTV, 1997) and the production of a biogas, mainly methane (55-70%) and carbon dioxide (25-40%), with traces of hydrogen and of H2S, (Mata-Alvares, 2003). Methane can be developed in the form of energy (boiler producing of heat or electricity). At the same time the anaerobic micro-organisms consume little energy, which involves a limited production of muds limited (3 to 20 times lower than an aerobic treatment), (Bitton, 1994). Indeed, the micro-organisms use only approximately 10 to 15 % of the energy of the substrate for their growth (Trably, 2002 and Moletta, 1993), the remaining being used for the production of biogas. Finally, anaerobic digestion allows a reduction of the pathogenic micro-organisms.

Anaerobic digestion consists of sludge fermentation, under strict anaerobic conditions. It is made up of four stages: hydrolysis, acidogenesis, acetogenesis and the methanogenesis. To achieve an anaerobic digestion, it is necessary that the reaction kinetics for the consumed or produced component is balanced. The general diagram of anaerobic digestion is presented on Figure 1 (Edeline, 1997)**.**

Fig. 1. Diagram of trophic chain of the methanogene and its various stages (Edeline, 1997)

Production of Biogas from Sludge Waste and Organic Fraction of Municipal Solid Waste 155

However, the partial hydrogen pressure has a great influence on the fermentation route where a low value encourages the fermentation to acetate and hydrogen is favoured

The acetogenic step allows the transformation of the acids, resulting from acidonenic step to acetate, and carbon dioxide, by the action of the acetogenic bacteria. This operation is

The mehanogenic step consists of the transformation of acetate, hydrogen and carbon

There are other minor routes which have a low importance. In the anaerobic digesters, approximately 60 to 70% of methane are produced by the Aceticlastic methanogens routes

The growth of methanogens bacteria is slow: 3 days in 35°C (Schink, 1997). As they are the most sensitive micro-organisms of the ecosystem, they govern the total kenetics of the process (Ramsay & Pullammanappallil, 2001). Moreover, they are sensitive to the presence

During the methanogenic phase, the products of fermentation such as acetate and H2 / CO2 are converted into CH4 and CO2 by methanogenic bacteria. Methanogenes bacteria can grow directly on H2 / CO2, acetate and all other compounds with only one carbon such as

The methanogenic step is influenced by the operating conditions of the digester, such as temperature, hydraulic loading rate, organic loading rate, and the influent substrate

The conventional anaerobic digesters operate as semi continuous, continuous or closed. The operations in semi continuous or continuous are preferable because the maximum growth rate can be obtained by controlling the effluent rate. In the closed system, a balance cannot be obtained while the concentrations of the components in the digester change with time

The choice of the type of digester used is related to treated waste characteristics. Solid waste and sludge are mainly treated in digester with continuous flow (CSTRs), whereas soluble organic waste is treated by a use of biofilm systems such as the anaerobic filters and

In the systems of biofilm the biomass is maintained in the aggregates of the biofilm/granule where the solid retention time (SRT) is much higher than the hydraulic retention time

fluidized bed digesters with ascending flow (UASB) Smith & al., 2005).

dioxide into methane. For that, there are two main system routes:

2. Hydrogenotrophic methanogens: CO2 + 4 H2 ⇔ 2 H2O + CH4

1. Aceticlastic methanogens : acetate + H2 ⇔ CO2 + CH4

formate, methanol and the methylamine (Puñal & al., 2003).

(Thauer, 1977).

**2.3.3 Acetogenesis** 

**2.3.4 Methanogenesis** 

of inhibitors such as VFA.

composition (McHugh & al., 2003)**.**

(Karakashev & al., 2005).

**3. Types of digesters and applications** 

(Oles, 1997).

carried out by different types of bacteria.
